Legionnaires’ disease has made headlines again after seven people living in Manhattan were diagnosed with the disease over just 11 days, according to officials at the New York City Health Department.

One of the patients — who had underlying health conditions and was in their 90s — died after contracting the disease.

Reported cases of the disease have skyrocketed since 2000, and just this year the disease made headlines after it infected a New York City police officer, four Florida gym goers, and two newborns in Arizona.

First discovered in 1976, Legionnaires’ disease, or Legionellosis, was catapulted into headlines after it infected 221 people attending a conference of American Legionnaires at a Philadelphia hotel. Of those infected, 34 people died.

Thousands are infected every year and out of those who become sickened by the bacteria, an estimated one 1 in 10 will die, according to the Centers for Disease Control and Prevention (CDC).

What causes Legionnaires’ disease?

In the decades after the 1976 outbreak researchers have learned much more about the naturally-occurring, potentially life-threatening Legionella bacteria that causes Legionnaires’ disease. The microorganisms are dangerous when they multiply in a water supply and become aerosolized into a breathable mist.

Cases are more commonly reported during the summer and early fall, when temperatures increase habitats that are more favorable to the bacteria. It’s also the time that more air conditioners are turned on, and people are visiting bodies of water where the bacteria may live naturally.

The disease is caused when the bacteria infect a person via inhaled droplets. The disease usually results in pneumonia-like symptoms, but can also include aches, pains, and diarrhea. It cannot spread from person to person.

In the 1976 outbreak it was the air conditioning system that aerosolized the water leading to the mass number of infections. But decorative fountains, cooling towers, and even grocery produce misters have also been linked to the bacterial infection in the past.

Dr. Frank Esper, assistant professor, and pediatric infectious disease specialist at Case Western University School of Medicine and Rainbow Babies and Children's Hospital in Cleveland, said that treating the water to kill the bacteria can also be tricky.

“The problem with Legionella is that it’s resistant to chlorine and so while they do try, you can't chlorine the problem away,” Esper explained to Healthline.

‘Puzzling’ increase

Despite decades of research, scientists remain unsure why the infections have been rising in the Unites States.

From 2000 to 2015 the number of estimated cases increased four-and-a-half times with approximately 6,000 cases of the disease reported in 2015, according to the CDC.

The reason for this drastic increase is not fully understood. One likely factor is that testing for the disease has become more commonplace, according to Dr. William Schaffner, an infectious disease expert at the Vanderbilt University School of Medicine.

“Legionnaires’ may account for 1 to 10 percent of all pneumonia cases,” Schaffner told Healthline. Among the possible “reasons that there may be an increase in reported Legionnaires’ is that doctors are using a widely available urine test ... you get the urine, and you detect remnants of the bacteria in the urine. It's much easier than getting a specimen of sputum.”

The CDC reports that the rise in cases could be related to a number of factors including a more vulnerable population, aging infrastructure, and a warming climate. Less invasive testing via urine instead of cultured bacteria could also be a reason that more cases are being reported.

However, none of these factors have yet definitively been linked to the increasing number of Legionnaires’ disease cases.

Locating a source of a disease outbreak can be difficult since the incubation period is between two to 10 days after exposure, and many healthy people will not exhibit any symptoms after exposure.

In addition to air conditioning units, Legionnaires’ disease outbreaks can occur from fountains or even the produce misters at grocery stories. In Arizona, two newborns were diagnosed with the disease after being born via water births in tubs.

Healthcare centers were found to be particularly adept at causing outbreaks, according to a report from the CDC published this June. After reviewing 27 Legionnaires’ disease outbreaks investigated by the CDC, the study authors found that these centers accounted for 57 percent of outbreak-associated infections, and 85 percent of outbreak-associated deaths.

While outbreaks can be challenging, Schaffner said tracking the origin of Legionnaires’ diseases not associated with an outbreak can be even more difficult.

“About 75 percent of cases are what we call sporadic, they occur here or there,” he said. “About 25 percent occur in clusters or outbreaks.”

Without additional cases found in an outbreak, narrowing down a single source of infection can be difficult or nearly impossible.

“We are all very puzzled by sporadic cases,” said Schaffner.

Stopping Outbreaks

To stop clusters of Legionnaires’ disease from spreading, health officials have focused on improving water management to stop bacteria from growing in the first place.

In the June CDC report, the researchers found that that 85 percent of outbreaks could have been stopped with more effective water management. This means keeping water temperature either above or below the warm temps favored by Legionella, keeping the water moving so it doesn’t stagnate, and adequately disinfecting the water.

Additionally, stopping corrosion and “biofilm” build-up in the water management system is key to keeping the Legionella from flourishing.

Getting the right diagnosis

Correctly identifying Legionnaires’ disease early is key to saving lives. While the test is relatively simple to run, many doctors may not think to test for this atypical cause of pneumonia, according to Schaffner.

“The trick is to think about it, because the conventional treatment for pneumonia doesn't work against Legionella,” Schaffner explained.

Those most at risk for infection are people with underlying health issues, people over 50 years of age, and those with compromised immune systems. Current or past smokers are also at higher risk.

In the CDC report the study authors emphasized that getting a quick diagnosis is also key to stopping an outbreak of Legionnaires’ disease from worsening.

“Legionnaires’ disease is clinically indistinguishable from other causes of pneumonia; a failure to diagnose a healthcare-associated case could result in a missed opportunity to prevent subsequent cases,” the authors wrote.

Esper said Legionnaires’ often presents as an “atypical” pneumonia. Since pneumonia can be especially deadly in older people, Esper said there is no time to waste in getting the them the right treatment.

“You don't give yourself a lot of leeway, you want to hit it, and hit it hard,” Esper said.